A transmission line balun is a passive device used as an interface between a balanced network and an unbalanced network. The balun is commonly used to convert an unbalanced radio frequency (RF) signal source to a balanced signal. However, a problematic side effect of this conversion involves unavoidable coupling between the balun and a nearby ground plane. This phenomenon will be referred to as parasitic mode propagation.
In parasitic mode propagation, some of the energy from the RF signal source propagates between the balun and the ground plane. A narrowband disappearance in frequency response, known as a suckout, occurs at signal frequencies in which the balun has an effective electrical length of (2N+1)90xc2x0 between the unbalanced port and the balanced port, where N is any integer. At these frequencies, the RF signal source is effectively shorted to ground and causes a narrowband suckout.
Prior art solutions adjusted the length of the conductive segments within the balun to shift the suckout outside the frequency band of interest. However, changing the segment lengths can cause the phase performance, match loss, and/or insertion loss of the balun to suffer. Furthermore, the suckout is only shifted to a different frequency bandxe2x80x94it is not completely eliminated.
In accordance with a preferred embodiment of the present invention, a junction between the conductive segments connected to the unbalanced port on the balun is connected to ground with a resistor to eliminate suckout due to parasitic mode propagation. The selected junction is a virtual ground of the balun, so the presence of the resistor does not degrade the balun performance. The resistor dissipates the energy from the unwanted signal coupling due to parasitic mode propagation, thus preventing the 90 degree phase delay that would short out the RF signal source. The value of the resistor is selected to facilitate maximum termination of the parasitic mode propagation. With all of the parasitic mode propagation terminated, no undesired reflection of energy can occur and the narrowband short circuit seen at the input is completely eliminated.
Further features and advantages of the present invention, as well as the structure and operation of preferred embodiments of the present invention, are described in detail below with reference to the accompanying exemplary drawings. In the drawings, like reference numbers indicate identical or functionally similar segments.